Large electric power consuming facilities such as oil refineries, steel mills and shopping complexes are major loads to a power transmission or distribution system. For computer-simulation based power system planning and operation studies, it is critical to model such loads properly due to their large power demands and complex responses to power disturbances. Inadequate models may result in a poorly designed power supply system or costly system investment decisions.
There is, however, a great challenge to establish adequate models for large facilities. For power system planning studies, basic information such as circuit diagrams, load composition and motor sizes etc. of the future industrial or commercial facilities are often not available. So it is extremely difficult to develop models of future facilities for long range planning studies. For power system operation studies, the facilities of concern have been built and are operating. But there are still difficulties to construct adequate models for them or ascertain the validity of the developed models due to confidentiality restrictions of the facilities and other constraints.
Modeling of electric loads for power system studies is of great interest to the power industry. Research has been directed on forecasting the growth of loads in a neighborhood, such as for example U.S. Pat. No. 6,865,450 (B2) and U.S. Pat. No. 6,577,962 (B1). Another type of frequently-reported work is how to estimate the responses of the loads, served by a substation, to the changes of substation supply voltages or currents. Some research has been conducted on how to represent a group of induction motors using an equivalent motor model. However, very limited progress was made on how to model major facilities.
For bulk power system studies, a load generally refers to the collective real and reactive power demand of various distribution feeders connected to a substation. Proper representation of the dynamic responses of such loads is very important for power system stability studies. Due to lack of adequate data, load is probably the most difficult component in a power system to model properly. As such, a lot of research has been conducted in the area of load model development. The most representative work can be classified as heading in two different directions. The first direction uses the information of load composition and associated component models to compute the load parameters. This approach is theoretically sound but has a number of implementation problems. For example, how do we classify the loads into different types that have similar voltage-dependent characteristics? What are the valid models for different load types and at different time and seasons? Such difficulties have led to the second direction of research—direct measurement of load parameters. These methods have to rely on voltage disturbances to be successful. Unfortunately, disturbances suitable for load parameter estimations are so few and irregular. The lack of representativeness of the estimated parameters has severely limited the use of the measurement-based methods.
Loads can be classified into two categories based on their ownerships. The first category can be called aggregate loads. These are the collective power demands of various loads owned by different utility customers and served by the same substation. Examples are the power distribution feeders connected to a substation. The load modeling research works reviewed earlier are mainly developed for aggregate loads. The second category is the large industrial (and sometimes commercial) facilities owned by one customer and supplied by a (often dedicated) substation. Examples of such loads are oil refineries, steel mills and large airports, which are called facility loads in this document. Such loads typically consume large amounts of power (e.g. >50 MW), have complex and unique responses to power system disturbances. Unfortunately, the load modeling methods developed for the first category has limited use to the facility loads.
Facility loads present major loads in a power system. It is critical to model them properly due to their large power demands and complex responses to system disturbances. A common situation encountered by utilities planners can be described as follows: a manufacturer contacts a utility company and plans to develop an X-type industrial facility in a particular location. The facility needs about Y-MW power and could be in service in several years. It is common that basic information of the future industrial facility such as single-line diagrams, load composition, loading factors etc. is not available. The utility, however, must include the model of the future facility in its planning study since the load can be hundreds of MWs. For power system operation studies, the facilities of concern have been built and are operating. But there are still difficulties to construct adequate models for them or ascertain the validity of the developed models due to confidentiality restrictions and other constraints.
Limited research efforts were reported to address this load modeling gap. Some attempts have been made to describe a method of creating equivalent models for industry facilities assuming the electric structure of the facility is known. In some cases, assumptions are made that an industrial load consists of about 76% small and large motors, and 24% static loads. A few utility companies have adopted very crude approaches such as the WECC modeling guide. This guide suggests that the system load can be modeled as 80% static loads and 20% induction motors. The load class mix includes residential, commercial, industrial and agricultural loads. If this approach is used, an oil refinery will have a similar response to that of a mining operation in power system dynamic studies. This is clearly not acceptable when more and more accurate models are being developed for aggregate loads.
There is a need for a systematic method to address the deficiencies in modeling facility loads.